This invention relates generally to traction vehicles such as locomotives that have thermal prime movers on board, and it relates more particularly to fuel saving means for protecting the prime mover from abnormal wear when idling.
Large self-propelled traction vehicles such as locomotives commonly use a thermal prime mover to drive an electrical transmission comprising generating means for supplying electric current to a plurality of direct current (d-c) traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the prime mover (typically a 16-cylinder turbocharged diesel engine). When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of the traction motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes full engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to full output. But the load on the engine must not exceed whatever level of power the engine can develop. Overloads can cause premature wear, engine stalling or "bogging," or other undesirable effects. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and maximum, and so that the engine develops, whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control console located in the operator's cab of the locomotive. (In addition to the eight conventional power notches, the handle has an "idle" position and a "shutdown" position).
The position of the throttle handle determines the engine speed setting of the associated governor. In a typical governor system, the output piston of an electro-hydraulic device is drivingly connected, via a mechanical linkage, to a pair of movable fuel pump racks which in turn are coupled to a plurality of fuel injection pumps that respectively meter the amounts of fuel supplied to the power cylinders of the engine. The governor compares the desired speed (as commanded by the throttle) with the actual speed of the engine, and its output piston moves the fuel racks as necessary to minimize any deviation therebetween.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,000) are realized. Under normal conditions the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipments.
The output power (KVA) of the main alternator is proportional to the product of the rms magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the magnitude of excitation current in the alternator field windings. For the purpose of accurately controlling and regulating the amount of power supplied to the electric load circuit, it is common practice to adjust the field strength of the traction alternator to compensate for load changes and minimize the error between actual and desired KVA. The desired power depends on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
In practice the above-summarized system of controlling a diesel-electric locomotive also includes suitable means for overriding normal operation of the system and reducing engine load in response to certain temporary abnormal conditions, such as loss of wheel adhesion, low pressure in the lubricating oil system or the engine coolant system, or a load exceeding the power capability of the engine at whatever speed the throttle is commanding. This response, which is generally referred to as "deration," helps the locomotive recover from such conditions and/or prevents serious damage to the engine. In addition, the excitation control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits.
The above-summarized locomotive in practice will often be at rest with its engine running, its throttle in idle position, and its main alternator developing no power (i.e., zero traction load). The regular idle speed of a locomotive engine is usually high enough to enable all engine-driven auxiliary equipment to function properly if operative while the locomotive is at rest. More particularly, it is high enough to assure that the pressure in the engine cooling system (which includes an engine-driven water pump and a plurality of radiators) is sufficient to circulate the coolant through the radiators if required. A regular idle speed of approximately 450 rpm is typical.
To conserve fuel while the locomotive is at rest with the engine idling, it is a known practice to reduce engine speed below the aforesaid regular idle setting (e.g., to a preselected "low idle" speed such as 385 rpm) so long as the engine coolant is relatively warm. But if the temperature of the coolant were to drop below a predetermined low limit (e.g., approximately 160.degree. F.), the engine is automatically returned to its regular idle speed, thereby producing more heat. Persons skilled in the art will understand that the operating temperature of a diesel engine needs to be above some minimum point for two different reasons: (1) engine fuel consumption, at any given idle speed, tends to vary inversely with temperature (increasing approximately 7% for each 10.degree. F. decrement in a 16-cylinder, 4,000 horsepower engine); and (2) sulfur in the fuel tends to corrode the engine cylinder liners at an unacceptably rapid rate when the coolant temperature is too low. Corrosive liner wear can be controlled by running the engine at a higher idle speed and/or by adding electric heaters so as to warm up the engine coolant. Since fuel consumption increases with engine speed, it is obviously desirable to minimize the time during which the engine has to idle at more than the low idle speed.